What duo you want to achieve? A bit more bass and speech clarity or Atmos also from the rear speakers (that Sonos to the chagrin of some also partly sends to the Ultra)?

We watch TV/Movies 50% of the time and stream music 50% of the time.  I am overall happy with the bass that I get from the subwoofer.  I am looking for just an overall better sound.  Not specific to bass.  Something that sounds good on regular broadcast TV,  movies and music.  I do however like the more snappy mid bass that I feel a lot of systems lack. Not looking for boomy sounds.  The genre of music that I listen to frequently would be 70’s and country.  There is a lot of good harmony and acoustical sounds as well as the snappier bass and not the boomy bass of todays music. When I do need the boomy sound, I feel that my SUB3 is producing that just fine.  I have read some reviews where people have indicated that when the ARC Ultra is on it’s own it has better bass then when a sub is added to it.  Not sure if by adding the sub, filters out some bass on the Ultra, or the sub is just drowning the Ultra out.  Just noticed that on some reviews.  End of the day, just looking to improve over all soundstage.  Hopefully that makes sense on what I am trying to accomplish.      

Since you are not complaining about speech clarity, watch a lot of movies and seem content with the bass you have now, I'd advise the Era 300's to get the real Atmos experience. 

Thank you!!!! 

I agree that you should choose Era300.

They can deliver height sound with Atmos/surrond so you get a slightly better experience with movies. Regular TV is rarely surround so you won't get much out of them. You will especially experience Era300 when you listen to music, and when you listen to Atmos music then you will really appreciate your Era300.

As for bass, you can adjust the settings a little and get some more bass. Loudness ON, Bass on Arc +1-3. I usually keep Sub at 0 but it's up to the listener.
I don't know if you can do a quick thrueplay on your system but I thought it helped on mine. (Ultra-Sub 4-Era300)

Thank you.  I will start looking into the ERA 300’s.